LIGHT-SABRE Hyperpolarizes 1-13C-Pyruvate Continuously without Magnetic Field Cycling

Nuclear spin hyperpolarization enables real-time observation of metabolism and intermolecular interactions in vivo. 1-13C-pyruvate is the leading hyperpolarized tracer currently under evaluation in several clinical trials as a promising molecular imaging agent. Still, the quest for a simple, fast, and efficient hyperpolarization technique is ongoing. Here, we describe that continuous, weak irradiation in the audio-frequency range of the 13C spin at the 121 μT magnetic field (approximately twice Earth’s field) enables spin order transfer from parahydrogen to 13C magnetization of 1-13C-pyruvate. These so-called LIGHT-SABRE pulses couple nuclear spin states of parahydrogen and pyruvate via the J-coupling network of reversibly exchanging Ir-complexes. Using ∼100% parahydrogen at ambient pressure, we polarized 51 mM 1-13C-pyruvate in the presence of 5.1 mM Ir-complex continuously and repeatedly to a polarization of 1.1% averaged over free and catalyst-bound pyruvate. The experiments were conducted at −8 °C, where almost exclusively bound pyruvate was observed, corresponding to an estimated 11% polarization on bound pyruvate. The obtained hyperpolarization levels closely match those obtained via SABRE-SHEATH under otherwise identical conditions. The creation of three different types of spin orders was observed: transverse 13C magnetization along the applied magnetic field, 13C z-magnetization along the main field B0, and 13C–1H zz-spin order. With a superconducting quantum interference device (SQUID) for detection, we found that the generated spin orders result from 1H–13C J-coupling interactions, which are not visible even with our narrow linewidth below 0.3 Hz and at −8 °C.


■ INTRODUCTION
Nuclear spin hyperpolarization of 1-13 C-pyruvate is a successful example of translating quantum technology into clinical practice. 1−3 Currently, dissolution dynamic nuclear polarization (dDNP) is the leading method to hyperpolarize pyruvate 4−7 and other small-molecule metabolites. 8−10 An alternative approach to dDNP is parahydrogen-induced polarization (PHIP), which is faster and simpler but less developed. 11,12 Hydrogenative PHIP has been used to produce close to unity hyperpolarization of pyruvate esters at high concentrations, 13−16 which was facilitated by the development of perdeuterated and 13 C-labeled pyruvate esters. 15−18 A nonhydrogenative PHIP variant is signal amplification by reversible exchange (SABRE), 19 which allows continuous (re)hyperpolarization of selected molecules. SABRE can also be used to hyperpolarize pyruvate and does not require hydrogenation. Instead, transient interactions with an Ircomplex are sufficient to hyperpolarize pyruvate without chemical modification, as depicted in Figure 1a. 20−23 The polarization transfer from parahydrogen (pH 2 ) to pyruvate was demonstrated at ultralow magnetic fields below 1 μT, where the pH 2 -derived protons and the 13 C nucleus of pyruvate become strongly coupled and anticrossings of nuclear spin energy levels occur. This principle is used in the technique known as SABRE in shield enables alignment transfer to heteronuclei, SABRE-SHEATH. 24−28 After hyperpolarization in SABRE-SHEATH experiments, the magnetic field is typically elevated to a few Tesla for detection. An alternative approach to transferring the pH 2 spin order to 1 H, 13 C, or 15 N is to use RF pulse sequences at a constant field; 29−35 however, this approach has not yet been demonstrated for pyruvate.
Here, we demonstrate continuous hyperpolarization of 1-13 C-pyruvate using RF pulses at a magnetic field of 121 μT (approximately twice Earth's field). The gained hyperpolarization levels are as large as those obtained with SABRE-SHEATH under otherwise identical conditions: −8°C, 51 mM pyruvate, 5.1 mM Ir-complex in MeOH, and ∼100% pH 2 at ambient pressure. The observed hyperpolarization is on the order of ∼1.1% averaged over free and bound species. Note that at −8°C, only bound pyruvate was observed, and polarization was estimated to be ∼11%. 22 We also demonstrate that three different types of spin orders can be generated: transverse (x), longitudinal (z), and two-spin order (zz). First, transverse (x) polarization can be generated along an applied, on-resonance RF field. This was the most efficient mechanism for polarizing the 13 C nucleus of 1-13 C-pyruvate studied here. Second, we demonstrate that z-magnetization can be generated by applying a weak RF field slight off-resonance. This mechanism is somewhat less efficient than on-resonance irradiation. We also describe the creation of zz-order on 13 C− 1 H spin pairs in 1-13 C-pyruvate for the on-resonance case.
Since the experiments were conducted at relatively low temperatures of −8°C, pyruvate exchange is strongly suppressed. 21,22 As in previous work, here, we also observe that hydrogen exchange, on the other hand, is still highly efficient. Figure 1b,c illustrates possible exchange mechanisms that allow for hydrogen exchange without pyruvate exchange under the assumption that hydrogen exchange is an associative mechanism, as supported by previous work. 36,37 We hypothesize that the pyruvate-SABRE system enables efficient hydrogen exchange because the pyruvate binds in a bidentate fashion, which allows for hydrogen exchange via partial dissociation of pyruvate (Figure 1b), opening a binding site for parahydrogen without pyruvate exchange all the way into its free form. A less likely alternative is depicted in Figure 1c, where DMSO dissociation allows for H 2 association and exchange. ■ METHODS Pulse Sequences. Since the sample degrades over time ( Figure S1, Supporting Information), the performance of the system was monitored with a SABRE-SHEATH sequence (Figure 2a) every 5−10 min.
For a direct comparison between the methods SABRE-SHEATH ( Figure 2a) and low-irradiation generation of high-Tesla (LIGHT)-SABRE (Figure 2b), the hyperpolarization time t hyp was varied. After the SABRE-SHEATH phase, the use of a 90°pulse flipping 1 H and 13 C spins enabled simultaneous observation of longitudinal signals for 1 H and 13 C, whereas after the LIGHT-SABRE phase, no additional excitation was required for detection of the transverse magnetization. LIGHT-SABRE scheme of the SABRE-reaction to polarize pyruvate, where chemical exchange and spin order transfer occur on the iridium-based polarization transfer catalyst. The LIGHT-SABRE RF pulse is applied close to the Larmor frequency of the 13 C nuclear spin. Hydrogen exchange remains active at lower temperatures, whereas pyruvate exchange is only active at elevated temperatures. In the present work, all experiments were performed at temperatures below 0°C. Panels (b) and (c) present possible exchange mechanisms that allow for hydrogen exchange without substrate exchange. The partial pyruvate dissociation mechanism shown in panel (b) seems more plausible than the co-ligand exchange mechanism shown in panel (c), while it cannot be ruled out at this time. Here, IMes is defined as 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; DMSO is added as a co-substrate to optimize exchange rates; red dotted arrows indicate polarization transfer between nuclei.
To compare with the theoretical SABRE model, the LIGHT-SABRE sequence was repeated multiple times, varying frequency offset from 13 C resonance Δν CW frq and amplitude ν CW A . Transverse 13 C magnetization generated in LIGHT-SABRE was measured without additional excitation ( Figure  2c(i), read-out scheme), while longitudinal magnetization was measured after a crusher gradient dephasing remaining transverse magnetization from the SLIC pulse followed by a 90°pulse on 1 H and 13 C (Figure 2c(ii)). The 1 H− 13 C longitudinal two-spin order was measured after a crusher gradient using a 1 H− 13 C SEPP sequence ( Figure  2c(iii), read-out scheme).
Sample. To demonstrate the feasibility of LIGHT-SABRE on 1-13 C-pyruvate, we prepared a sample consisting of 51 mM 1-13 C sodium pyruvate, 5.1 mM [Ir(COD)(IMes)Cl] SABRE precatalyst (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2ylidene; COD = 1,5-cyclooctadiene), and 18 mM dimethyl sulfoxide (DMSO) dissolved in methanol-H 4 . Close to 100% enrichment pH 2 was prepared using an in-house-built liquid helium pH 2 generator. 38 Spin Simulation. To simulate the SABRE experiment, we used the formalism developed for the linear exchange model. 39 Noting the critical difference, we assume that only H 2 exchanges, while the substrate stays bound to the Ir-complex. This can be expressed with the following chemical reaction

IrHHPyr
IrH H Pyr H exchange 2 where IrHHPyr refers to the active SABRE complex, and the prime symbol (′) is used to distinguish hydrogens before and after exchange. For this chemical exchange, the following generalized Liouville−von Neumann equation has to be solved numerically: is the superoperator of the direct product that adds pH 2 to IrPyr, Tr IrHHPyr HH is a superoperator that removes HH from IrHHPyr, resulting in a density matrix for IrPyr only, 1/τ Ir is the exchange rate of H 2 , and τ Ir is the lifetime of the complex.
The simulations were fitted to the experimental observation, varying τ Ir , and were scaled up to fit the experimental polarization values. Note that theoretical predictions result in an overestimation of the 13 C polarization level. Predicted polarization levels were multiplied by a factor of 0.38 for 13 C and 3.4 for 1 H to fit the experimentally observed polarization. The system parameters are detailed in Table S2, Supporting Information.
In this simulation model, we completely neglected the transient complexes presented in Figure 1b,c. Also, we assumed that pure pH 2 substitutes dihydrogens in the Ircomplex after each exchange event. Both effects were simulated before in more detail. 40 However, because very little is known about these evolution steps of the Ir-complex, we used the most straightforward exchange scheme given above.

■ RESULTS AND DISCUSSION
In the current demonstration, 1-13 C-pyruvate SABRE polarization was generated and observed at a constant magnetic field of B 0 = 121 μT. The Faraday coil benchtop NMR spectrometers at such low field strengths are not sensitive, so we opted for using a superconducting quantum interference device (SQUID)-based ultralow field spectrometer. 38,41,42 Since SQUIDs are broadband detectors, signals from 1 H and 13 C can be detected simultaneously. The noise level of the employed SQUID detector is on the order of 1 fT Hz −1/2 , resulting in signal-to-noise ratios of above 8000 in a single shot when detecting the hyperpolarized 1-13 C-pyruvate. The SQUID NMR system sits inside a three-layered shielding chamber for DC and RF fields with a residual magnetic field below 10 nT at its center. The capability of field cycling makes it a versatile system for all kinds of in vitro SABRE experiments.
Out of the existing spin order transfer sequences, LIGHT-SABRE 33 is probably the simplest because it only requires a single, weak, and constant irradiation close to the Larmor  13 C magnetization, no further pulses are needed (i). For observation of longitudinal magnetization of 13 C and 1 H, the transverse magnetization must be dephased via a crusher gradient before a 1 H and 13 C 90°excitation pulse (ii). The longitudinal 1 H− 13 C two-spin order was measured after a SEPP sequence (iii). Note that in SEPP, there is a 90°pulse only on 1 H, whereas the 180 o pulse flips 1 H and 13 C spins.
The Journal of Physical Chemistry C pubs.acs.org/JPCC Article frequency of the targeted nucleus, also referred to as spin-lock induced crossing (SLIC) pulse. 43 The main idea behind SLIC and LIGHT-SABRE is that a correctly set continuous wave (CW) RF pulse matches two energy levels of the hydride−substrate spin system. Consequently, the spin alignment of pH 2 evolves into the polarization of the target spins in the substrate. In contrast, SABRE-SHEATH uses B 0 to achieve the same polarization transfer effect, adjusting and matching the energy levels by setting B 0 . The advantage of using B 1 , instead of B 0 , for spin order transfer is that field cycling becomes obsolete, and hyperpolarization could be produced at any magnetic field. In the specific case of 1-13 C-pyruvate SABRE, the J-coupling network is dominated by the hydride−hydride coupling J HH of ∼−10.5 Hz 23 such that irradiation with a LIGHT-SABRE pulse on-resonance with the 13 C Larmor frequency and amplitude, ν CW A , set to ∼10.5 Hz is expected to give maximum hyperpolarization transfer to the 13 C spin.
When LIGHT-SABRE or related methods are used at much higher, multi-Tesla magnetic fields, then pH 2 bubbling must be interrupted during LIGHT-SABRE irradiation to avoid B 0 and B 1 inhomogeneities. At the ultralow magnetic fields used here, B 0 = 121 μT, pH 2 can be bubbled through the solution during irradiation and acquisition of the free induction decay (FID) without any noticeable decrease in B 0 or B 1 field homogeneity because (a) susceptibility effects scale with the magnetic field strength and (b) the audio frequency irradiation can easily be performed with large and homogeneous Helmholtz coils. In our experiments, the full width at half maximum (FWHM) of 13 C lines measured was below 0.3 Hz.
The low magnetic field is also beneficial for 1-13 C-pyruvate LIGHT-SABRE because it preserves the singlet spin state of pH 2 after association with the Ir-complex. Although LIGHT-SABRE was proposed and used at high magnetic fields to polarize pyridine, 33 nicotinamide, 44 and 4-amido pyridine, 34 in such cases, pH 2 -derived protons (IrHH) had the same chemical shifts. This is not the case for pyruvate, where the hydride−hydride chemical shift difference is 2 ppm. 21 To compensate for this problem, one typically has to use strong RF pulses to lock the protons in a singlet state, 45 which will also alter the LIGHT-SABRE conditions. 46 At the low magnetic fields employed here, the hydride−hydride chemical shift difference is much smaller than their mutual J-coupling interaction of J HH ∼−10.5 Hz ≫ Δν HH , which is the requirement for maintaining a singlet state without spin locking.
In the present work, LIGHT-SABRE produces a range of different spin orders in the polarized substrate (1-13 Cpyruvate), including the following: (1) transverse polarization (S x or S y ), which is generated parallel to the LIGHT-SABRE CW pulse in the rotating frame, when applying the LIGHT-SABRE pulse on resonance, (2) longitudinal (S z ) or zpolarization of the 13 C nucleus (S), which is the result when applying LIGHT-SABRE slight off-resonance, and (3) the presence of additional spins (CH 3 group) (I) that can result in additional polarization of ( 13 C− 1 H)-zz two-spin order (S z ·I z ) or zz-polarization. To detect and distinguish these three different spin orders, 13 C x-polarization (Figures 3 and 4), 13 Figure 4). Also note that the 13 C polarization was averaged across free and bound pyruvate, and the bound polarization was estimated to be 10 times larger.
The Journal of Physical Chemistry C pubs.acs.org/JPCC Article tested. The individual pulse sequences are fully described in Methods ( Figure 2).
An iteration between experiments and simulations found the optimal RF conditions for the LIGHT-SABRE experiment. Finally, the simulations were fit to the experimental data, showing good agreement as discussed below. The MOIN-spin library 40 was used to simulate all SABRE experiments. All scripts are available in the Supporting Information.
First, the generation of transverse (x) 13 C magnetization was investigated as a function of the B 1 amplitude ν CW A and the frequency offset from the 13 C Larmor precession frequency Δν CW frq of the CW pulse with a fixed hyperpolarization time t hyp = 10 s at B 0 = 121 μT (Figure 3). Here, t hyp = 10 s implies a 10 s-long LIGHT-SABRE pulse. We note that pH 2 constantly bubbled through the solution even during acquisition. Under these conditions, the hyperpolarized transverse magnetization showed the highest enhancement at Δν CW frq = 0 Hz and ν CW A ≅ 1.1 μT. This amplitude of the CW pulse corresponds to a 13 C B 1 Larmor frequency of 11.8 Hz, which is close to the J HH = −10.5 Hz coupling as theoretically predicted.
The simulations indicated that the width of the LIGHT-SABRE polarization in the ν CW A dimension is mainly dependent on the lifetime of the active Ir-complex τ Ir . For the experiments performed at −8°C, the best fitting was achieved with τ Ir = 31 ms, where this lifetime is primarily dominated by hydrogen exchange at low temperatures. The acquired spectra did not allow us to distinguish between free and Ir-bound 1-13 Cpyruvate because the chemical shift difference for pyruvate in bulk and that coordinated to Ir is about 1.3 ppm, which corresponds to ∼1 mHz at B 0 = 121 μT, which is below the FWHM of the corresponding 13 C lines. As described previously, 22 no significant hyperpolarization transfer to free pyruvate occurs at these low temperatures. Therefore, in the present work, the observed hyperpolarization is primarily on the catalyst-bound pyruvate.
In previous SABRE research, the SABRE activity was stopped by the addition of bidentate ligands 2,2-bipyridine or 1,10-phenanthroline to activated Ir-complexes, which resulted in complete suppression of substrate exchange. 47 However, hydrogen exchange remains active as it was later demonstrated that the substrates on the Ir-complex can still be polarized. 48 In the present work, with bidentate pyruvate binding, we have a similar situation. At low temperatures of −8°C , there is no observable pyruvate dissociation on the NMR timescale of several seconds; 22 however, the complex-bound pyruvate continues to be polarized. Considering that all evidence points to the need for substrate dissociation before hydrogen exchange, 37,49 we needed to introduce alternative possibilities for this process: partial pyruvate dissociation ( Figure 1b) and co-ligand elimination (Figure 1c). To deduce which method is more probable, DFT simulations similar to the ones made for the prototypical Ir-complex with pyridine 36 or more detailed exchange measurements are needed. The critical conclusion from this discussion for the present work is that the observed signals predominantly stem from catalystbound pyruvate.
Despite the excellent resolution with a 13 C FWHM) of 0.3 Hz, we could not identify any hydride− 13 C J-coupling constants. Accordingly, we used the J HC = 0.06 Hz value for the simulations, which is below the FWHM of the 13 C lines of 1-13 C-pyruvate (see the Supporting Information for further details). This value is two orders of magnitude below the one used before to simulate the spin evolution of this system (5 Hz in ref 23). Note, however, that if indeed the lifetime of complex τ Ir is about 31 ms as was estimated here (Figure 3), then due to fast exchange, the lines will collapse in one and no J-coupling interactions below ∼1/τ Ir ∼ 30 Hz (and 1/T 2 *) will be resolved (see the example in the Supporting Information, Figure S3).
Theoretically, the creation of transverse x-polarization, when applying a B 1 field on resonance, along the x-axis, can be rationalized by examining the following portion of the governing Hamiltonian as fully derived in the Supporting Information:  Under the condition that the frequency offset, Δν CW frq = 0, and the amplitude of CW are exactly on resonance with the Jcoupling interaction, i.e., ν CW A = −J HH , the difference of the diagonal elements becomes zero and the off-diagonal element, ΔJ CH /4 (the difference between the two hydride− 13 C Jcouplings), can efficiently couple |S 0 X − ⟩ to |T 0 X + ⟩ and drive spin alignment from the pH 2 into polarization along the applied B 1 field on the 13 C nucleus.
To benchmark the new field cycling-free LIGTH-SABRE experiment vs the established SABRE-SHEATH method, we measured the build-up of hyperpolarization using both methods under otherwise identical conditions (Figure 4). SABRE-SHEATH seems to have a slightly faster build-up rate of (15.8 ± 2.9) s than LIGHT-SABRE with a build-up rate of (25.8 ± 2.1) s for 13 C polarization. Remarkably, LIGHT-SABRE gives a hyperpolarization level that is comparable to that of SABRE-SHEATH. The achieved hyperpolarization level is 1.1% when averaged over free and bound pyruvate. However, considering that the signal predominantly stems from bound pyruvate, which is at a 10-fold lower concentration, the hyperpolarization was estimated to be 11% for the bound pyruvate. The fact that both LIGHT-SABRE and SABRE-SHEATH on the same system allowed for close to identical signal enhancement indicates that both techniques are of the same polarization transfer efficiency for the pyruvate SABRE system.
Next, we explored the creation of z-polarization with LIGHT-SABRE ( Figure 5). The advantage of creating zpolarization is that the spins will not dephase as quickly once created, also because they are subject to T 1 decay and not to T 1ρ effects, such that it may be easier to build up more magnetization simply by applying longer LIGHT-SABRE pulses. The observation of 13 C z-polarization requires a 90°p ulse after CW. To avoid any contribution to the signal from transverse components, we also implemented a magnetic field gradient, which purposefully dephased all transverse spin orders generated with LIGHT-SABRE before applying a 90°p ulse to assess the longitudinal spin order only. Two extrema at Δν CW frq = ±6.3 Hz were observed for longitudinal 13 C magnetization both in experiments and simulations, which were found to match nicely ( Figure 5). 13 C z-polarization on the order of 0.04% was observed, which remained below 0.12% achieved for x-polarization with identical hyperpolarization time t hyp . Note that in Figures 3  and 5, the hyperpolarization time was 10 s, while the maximum polarization of 1.1% (averaged across free and bound; catalystbound p ≈ 11%) was reached at about 80 s (Figure 4).
Theoretically, the creation of z-polarization, when applying a B 1 field slight off-resonance Δν CW frq , can be rationalized by examining the following portion of the governing Hamiltonian represented in a rotating frame of reference with a tilted axis for the 13  . Then, the effective field experienced by 13 The basis for 13 C in this Under the condition that B 1 is applied such that ν eff = − J HH , the difference of the diagonal elements becomes zero, and the off-diagonal element can efficiently couple the states and drive polarization from the parahydrogen-derived singlet into partial z-polarization. As is fully detailed in the Supporting Information, the creation of z-polarization is the most efficient when sin(2θ) × sin(θ) is maximized, which occurs at "the magic angle" of θ ≅ 54.7°or for = tan( ) 2 . This theoretical finding is experimentally substantiated in Figure 5b, where the combination of offset Δν CW frq and amplitude ν CW A at values close to the magic angle gives the highest polarization levels.
Finally, to observe 1 H− 13 C zz-polarization, we excited only 1 H with a 90°pulse that resulted in the antiphase spectra. Alternatively, we also used the selective excitation of polarization using the PASADENA (SEPP) sequence. 50,51 SEPP converts the antiphase spectral lines ( Figure 6) into the in-phase spectra, which can be used for 1 H imaging, for example. As detailed in Figure 6, a good match between the experiment and the simulation was obtained for the creation of zz-polarization.

■ CONCLUSIONS
To conclude, a new method for the continuous hyperpolarization of 1-13 C-pyruvate was demonstrated, and 13 C polarization of more than 1% was obtained after weak irradiation at the 13 C Larmor frequency. Since pyruvate exchange is suppressed under the current experimental conditions of −8°C, the bound pyruvate hyperpolarization is estimated to be 11%. In this first proof-of-concept demonstration, the generation of x-polarization under onresonance conditions was higher than the direct generation of z-magnetization under slight off-resonance conditions. For the generation of x-and z-polarization, we provided the theoretical underpinning by exploring the states that are coupled. Furthermore, 1 H− 13 C two-spin order was revealed. In our experimental setup, similar polarization levels were obtained with LIGHT-SABRE and control SABRE-SHEATH experiments. Although the ultralow field system has a superior magnetic field homogeneity, giving linewidths of below 0.3 Hz, we could not measure the 1 H− 13 C J-coupling interaction due to fast exchange compared to the size of the interaction. Hence, even lower temperatures are necessary to observe it. This interaction is responsible for polarization transfer in SABRE-SHEATH and LIGHT-SABRE experiments. Furthermore, we discussed that pyruvate does not exchange into its free form at the employed low temperatures. However, the bound pyruvate is still polarized efficiently, implying that hydrogen still exchanges at a significant rate. With these insights, we proposed two mechanisms for the necessary H 2 exchange process: partial cleavage of pyruvate or axial co-ligand (DMSO) elimination. We suspect that the partial pyruvate dissociation is the more likely explanation and intend to substantiate this claim with future theoretical and experimental evidence.
In future extrapolation to spin systems that may have additional coupled spins, e.g., other 1 H, 2 H, or 31 P, SABRE-SHEATH would transfer polarization to all of them, 14,52−54 while LIGHT-SABRE transfers polarization from pH 2 only to the irradiated spins, making the process more focused. We also observed this effect, simulating the effect of adding the methyl protons to the system. Addition of the methyl protons in the simulations reduced SABRE-SHEATH polarization more than LIGHT-SABRE polarization (Table S2, Supporting Information). Importantly, LIGHT-SABRE can avoid spin order transfer to relaxation sinks such as quadrupolar nuclei (e.g., 2 H or 14 N), 55−57 which often pose a challenge in SABRE-SHEATH. 58 These considerations imply that for more complex spin systems, the LIGHT-SABRE technique can be expected to be more efficient than SABRE-SHEATH.
In our experiments conducted at B 0 = 121 μT, the Larmor frequency of 13 C was about 1300 Hz. Thus, the irradiation frequency of the present demonstration lies in the audio frequency range. This is to the benefit of the LIGHT-SABRE approach because a simple audio source like a sound card can generate the necessary irradiation for polarization transfer without heating up the sample. In the case of dDNP, microwave sources and microwave guides are necessary.
One of the limitations of LIGHT-SABRE and similar methods is that they are frequency-selective. Hence, the efficiency is reduced when B 0 field homogeneity cannot be maintained. At ultralow magnetic fields, constant pH 2 bubbling did not deteriorate B 0 homogeneity.
In this work, low temperatures (−8°C) were used to allow for more efficient polarization transfer to bound pyruvate achieved at the reduced hydrogen exchange rates. A consequence of the low temperatures is almost complete suppression of pyruvate exchange, which, in future experiments, could also be compensated by using higher pH 2 pressures. In the current system, we only had atmospheric pH 2 pressure available, and more polarization may be expected at higher pressures and faster flow rates. 59 Because the Jcoupling interaction of pyruvate with pH 2 is much smaller than 1/τ Ir , the apparent hydrogen exchange rate, it appears that more studies in the direction of optimization of the catalyst and co-ligand (here, DMSO) are necessary. At present, potentially effective strategies to harness the high degrees of bound pyruvate hyperpolarization are either temperature cycling as performed in the context of SABRE-SHEATH 22 or the addition of highly competitive ligands after hyperpolarization to displace the bound, hyperpolarized pyruvate from the catalyst.
Finally, we note that the used carben Ir-complex was primarily optimized for the polarization of pyridine-like molecules. 37,60 With novel optimized catalysts, the SABRE spin order transfer sequences developed might also be used to generate hyperpolarization at higher fields outside of the μT regime.
The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Theoretical background for LIGHT-SABRE; evaluation of polarization level; sequence parameters; effect of the number of spins on 13 C polarization; stability measurements; SABRE-SHEATH field dependence and optimal field condition; 13 C-SABRE spectra at the ultralow field and different exchange regimes; references (PDF) MOIN-spin library with all simulation scripts (ZIP)